Amplitude modulation



Patented Apr. 17., 1945 Maurice Artzt, Princeton, N. J., assgnor to Radio Corporation of'America, a corporation of Dela- Ware yApplication February 22, 1943, serial No. 476,713

14 claims. (c1. 17e-17:15)

My invention relates to the art .of amplitudel modulation and particularlyy to a method of .and means for amplitude modulating an'bscillator of the resistor-capacitor or RCtype. The invention will be described, for the purpose'of illustration, as applied to a facsimile system but it should. be vunderstood thatthe invention is of general application. l

One of the. objects of the invention is toprovide .a method ofV and means for modulating a carrier wave at the maximum modulating or keyingate that will produce a satisfactory signal at. the receiver.

Another 'object of the invention is to provide an improved method of and means for amplitude modulating or keying a resistor-capacitor oscillator so that the keying signal itself always,

determines the zero-phase starting position of the vcarrier wave.-

A furtherobject. ofthe invention is to provide a method of and means for keying a carrier wave at a high speed without the introduction of transient signals or direct-current components.

A still further object Aof the invention is .to provide an improved method of and means for lamplitude modulating an'oscillator of the resistor-capacitor type.

.A still further object of the. invention isto provide an improved. method of and` means. for amplitude modulating a sine wave oscillator by keying dots of Variable'length or duration.v

In a preferred embodiment of the invention the oscillator is' o f the RC' type having a feedback circuit in which a delay network shifts the phase of the oscillation frequency signal 180 degrecs. The output of the'oscillator' tube is fed into the delay network through a cathode-follower tube rwhereby `the amplitude vof oscillation depends upon the gain of the latter tube. The gain of the cathode-follower tube is controlled by means of a modulator tube to which` the modulating sig-nal is applied, the modulator tube anode and the cathode-follower tube anode vrbeing supplied with voltage through acominon anode resistor whereby anv increase in anode current in the modulator .tube causes a decrease in anode voltage on the cathode-follower tube, thus reducing its gain. The cathode of the cathodethemodulator tube. .If circuit values are properlyk selected, the decreased voltage drop acrossone cathode resistor is balanced by the increased voltage drop across the other cathode resistor and the potential v.of the lcathode-follower tubes cathode with respect to ground, isy notaiected by the. modulating Sgal;

Therefore, the modulating signal does not introduce anytransient. at they input of the delay network; it. merely controlsv the gain in the feedback loop, thus controlling the. amplitude ofthe sine Wave oscillation. .In the facsimile application of the invention which will be described, this amplitudey control takes the form of starting and stopping the oscillations., An important feature of the invention, that a keying. signal startsr an. oscillation cycle at its zero phase position and always allows. the oscillator to finish a complete oscillation cycle. once the cycle. has started. y

` In@ accordance with. another'feature of my Vinvention the oscillator is provided with an additional feed-back connection or network having substantialhf. the same time constant as" thev Figure 1 is a circuit,v diagram of a simplified form of theinvention; `lijigure. 2 is. a circuit dia- I -gram of one preferred embodiment `of the invention; Figure 3 is a group of graphs which are referred` to. in .explaining the invention; and Figure 4 is al circuit. diagram of another embodiment of the invention. Inthe several figures similar parts arev indicated by similar reference characters.

llfteferringto.1the oscillator tube'Tl lmay be la screen grid vacuum tube'lraving a cathode 1.0 a control. grid H, alscreen grid I'2 and an anode I3.. Cathode. biasingl maybe provided` by a resistor M shunted by a. capacitor I6. The anodel is direct-current connected tothe grid l1 of a cathode-followerftube. T2.- The tube T2 may be a triode having the grid H, an anode I8, and a cathode ISQ Anodevoltage is.. supplied to. the tube'sTll and T2 through resistorsl 2U and 2l., respectively'. l u

The oscillator isof the, well known resistorcapacitor or RCtype comprisingy adelay network i2,v .consisting of a .plurality ofsections of series resistors 23fand shunt. resistors'24.. The coupling into thenetwork 22 isthrOugh a cathode load resistor 2 6. 'lhev output .of the net-work 22 is fed backv to the grid Il lof tube Tl. through a. conductor 2l. The oscillator will oscillate at the frequency for ywhich the network 22 introduces a phase shift of 180 degrees. The oscillator output, which will be sinusoidal in wave form, may be taken off the cathode I9 through an output lead 28.

The gain of tube T2 is varied by means of a variable resistor device T3' which is connected in series with the anode resistor 2| and a re-y sistor 3| whereby elements 2|, T2 and 3| form a potentiometer across the plate voltage supply. Resistors 2| and 3| preferably have the saine value of resistance, this value preferably being less than that of the load resistor 2B. The value of resistor 26 should be about equal to the platecathode impedance of the tube T2 if the cathode I9 is to follow the grid I1 closely.

Now considering the operation of the modulating circuit, assume that the potential on the anode VI3 of tube TI (which is also the potential on the grid I1 of tube T2) is not varying, andaslsume that the resistance of T3' is decreased.

Then the increased current ow through 2|, T3 and 3| makes the anode I8 of tube T2 go less positive and makes the high. end of resistor 3| go more positive with respect to ground. Thus the current flow through the tube T2 and the cathode resistor 2-6 decreases vboth because of the lowered anode-to-cathodepotential and because of the tendency of the cathode `I9 to go more positive With respect to the fixed potential of the grid I1.

As a result of this decreased current flow through tube T2 and resistor 26, the voltage drop across the anode-cathode impedance of tube T2 is decreased and the voltage drop across resistor 26 is decreased. `At the same time the voltage drop across resistor 3| has increased. Therefore, the potential of the cathode I9 with respect to ground is unchanged by the change that was made in the resistance ofv T3 with'the result that no transient signal is impressed upon the network 22. The gain of the tube T2 is lowered, however, becauseof the lowering of the anodeto-cathode voltage.'

Assume now that the tube TI is oscillating whereby a sine wave voltage is present on the anode I3 and is impressed `on the gridv I1 of tube T2. Thepotential' of Cathode I9 Will fOlIOW the sine wave voltage on the grid I1, and the sine wave oscillation is thus impressed upon the network 22 and fed back to the grid II of the tube T I in the proper phase to reinforce the oscillations. The'magnitude of the oscillations will depend upon the gain of the cathode follower tube T2. Thus, if the anode-cathode voltage of T2 has been loweredxby a change in T3', the sine wave variations on the grid I1 will produce current variations in resistor' 26 that are of lower amplitude and less signal is fed back through network 22. tude of the sine wave oscillations in response to a reduction in the resistance of T3', with no introduction of transient voltages.

Fig. 2 shows a useful embodiment of the invention which differs from Fig. 1 in thatV the resistor T3 is a vacuum tube T3 which has its anode-cathode impedance variedby a modulating or keying signal. The grid'of tube T3 is connected through an input resistor 33 to a suitable biasing potential obtained from a potentiometer comprising resistors 34 vand 36. A lter capacitor 31 is connected across resistor `36. The keyingv signal amplitude modulates-the oscillator as previously described.

'In order to bring the oscillations upto full The result is a reduction in ampli-- amplitude substantially instantaneously in response to the application of a keying signal, there is provided a regenerative feed-back connection from a point on the plate resistor 2| of the tube T3 to a point on the last resistor 24 of the network 22 to introduce feedback of a derivative of the keying signal. This connection comprises a capacitor 38 and a conductor y39. 4The time constant of this feed-back connectionpreferably is substantially equal to the time of the oscillator build up or decay, that is, the time of one quarter cycle. Then a voltage fed back through 38--39 will build up to maximum in the same time an oscillation cycle builds up from Zero to maximum amplitude. The time constant may be adjusted by moving the tap 4I on resistor 24. The amount of feed-back may be adjusted by moving the vtaplI2 on resistor 2|.

coupling so that the direct current component is removed it will be shown in graph d. It isk evident that the signal of graph d wouldy print a medium gray almost all through this series of keying signals with only the long dot of the series printing at full amplitude.

With my invention, however, the resulting amplitude modulated signal is as shown in the graph e. While most of the short dots are slightly elongated, they are all printed at full amplitude and with denite spaces between them. This results from the fact that a carrier wave cycle always builds up from zero phase at the start of a keying dot or signal and from the fact that at least Aone full cycle of oscillation is completed regardless of when the keying signal terminates. Thus the modulated signal contains no directcurrent components and can'be passed through alternating-current ampliiiers without obtaining signal distortion of the type shown by the graph d.

Fig. 4 shows another embodiment of the invention in which a pair of diodes 5| and '52 are employed to limit the amplitude of the positive and negative cycles, respectively, of the carrier wave,.the amplitude limits being controlled by the tube T3 upon which the keying signals'are impressed. In ,this circuit the output of the tube TI is supplied directly to the' delay network 22 through a conductorY 53. The diode 5I is connected in series relation with the resistor 2|l and with its anode connected to the conductor 453, while the diode 52 is connected in series relation with the resistor 3| and with its cathodel connected to the conductor 53. As in the other embodiments of the invention, the resistors 2| and 3| preferably are of equal resistance'so that the application of the keying signal will change the potential at the cathode of diode 5I and the potential at the anode diode 52 like amounts,`

flow through resistorZI, the-tube T3 and the resistor 3 I.

This causes the cathode potential of f,

asvagvafzf the-oscillator inciudesan .additional Vfeed-back diode fil. to :gol in: thepositive directionf a certain amount and causes theafanodeptentialof diode.` 52Std ygoin sthei negative direction fallikel. amount. Thus thef diodesll and 52;are rendered non-5 conducting; until. the; :a'mplitudeofr voscillation at the anode :of .T l. Cand,..cn5conductor; 53a)- increases enough yto..overcome the said cathode: and-xanode potentialichanges. The amplitude increasepreferably occursin ,one-quarter `ofwaf: cycle, .after which'. time thediodes. 5l. and .52.fa'gain conduct current and load the :platefcrcuitflof tuberTl to reduce `*its gain andzhold thexamplitude torthe.y level determined'by thekeying,signal;.;, .v s: i. It will .be noted. thaty in.. Figzz the keying ,signal will not 'affect V the-potential of i .the conductor 53. leadinginto the network 22. providing resistors 2| and 3i are equalandfyprcviding' the.diodes;5| and 52; aresalilize.r Thereforegthiscircuit, likethe ones previously described -produces an amplitude modulated wavethatrcontains no directfcurrent component. 1,

y :Itfshould be.,understood thaty the'invention is` not limited to.; the;,.us,eo f modulating vsignals of thetype illustrated in Fig.l 3 for keying the loscillatorr on andof. -Sine waveor speech signal mod.- ulation, for example-may hejapplied to theoscil-J liOr.. v u, .-.-i,-

. -Ilclaim asmy invention: y y.:

1-. combination, an I oscillator :of l the.,v type4 including a delayv network, for reversing the yphase of the, feed-back. voltage, `means forchanging the amplitudeof theoscillations whichare fed into. said network, the input terminals of said net:

lwork;havingna.; certain direct-current potentialcomponent thereacross in the absence of changes y in saidramplitudeand means for simultaneously maintaining the direct-current potential component across the input terminals of said network substantially constant.

2, In combination, an oscillator of the type including a resistor-capacitor delay network for reversing the phase of the feed-back Voltage, said 1 network consisting of a plurality of recurring f maintaining the direct-current potential compo-f* nent across theinput terminals of said network substantially constant.

3. The invention according to claim 2 wherein the oscillator includes an additional feed-back connection which has a .time constant that is of the order of the time period of one quarter cycle of said oscillations.

4. An oscillator of the resistor-capacitor type comprising an amplifier tube having an input circuit and an output circuit', a feed-back connected between said output circuit and said inputcircuit, said feed-back circuit including a delay network for reversing the phase of the feed-back voltage, the input terminals of said network having a certain direct-current potential component thereacross in the absence of changes in the amplitude of the oscillations applied thereto, and

means connected between said output circuit and said delay network for controlling the gain of the 'circuit feeding into the delay network while simultaneously holding the direct-currentpotential across the input terminals of the delay network substantially constant.

5. The invention according to claim 4 wherein connection which has a .time constant that'is of theorder ofethe.-7 timel period of; one quarter cycle ofsaid oscillations; v l f 6. 1 I'Anioscillator `of the :resistor-capacitor.: type comprising .an vfampliii'er'-.tuhe rhazvingfan.` input, circuit and an output circuit, a feed-back circuit connected betweenrsaid output'circuit and said input circuit,said `feed-back circuit including a l trolling the;'gainwofthecircuit -feedingyinto the delay.1 network, ysaid last means includinga sec-t f 1 ond amplifier, tubefhavingeau linput circuit to.

delaynetworkfor reversingthe phase-of the; feed-- 'back voltage,:andmeans connected between said output` circuitrand saiddela-y networkfor con-z whicha modulating `signal maybe applied, and means including'anode andcathoderesistors for said second ampliei' tube forbalancing outthe vmodulating signalx at thefinput terminals ,of-the between said-anode resistor; andthe-outputend.

of said.delay` networkand .which has a time constant ofthe jorderoffthe build-uptime of the oscillations.generatedby the oscillator..

` An; oscillatorfgof the-r resistor-capacitorl jtype comprising anamplier tube. having an input circuit and an output; circuit,v afeed-back circuit connected-between said, outputcircuit and said input circuit, saidffeedback -circuit includ,-

ing. 'ai Adelay network fon-reversing 'the phase' of` the feed-backfvoltageandalso including arramplitude control ainplier` through which the oscillations are .fed. from rsaidoutput ,circuit ,tosaid delay rnet ,vvi; rk',1theinputgternfiinalsJ of said netrwork having a certain direct-current potential component thereacross in the absence of changes in the amplitude of ,the oscillations applied thereto, and means for controlling the gain oi said amplitude control amplier in accordance with a .modulating signal while simultaneously holding the direct-current .potential across the input. terminals of the delay network substantially constant.

9. .The invention according to claim 8 wherein the oscillator includes an additional feed-back connection which has a time constant that is of the'order of the time period of one quarter cycle of said oscillations.

10. An oscillator of the resistor-capacitor type comprising anamplier tube having an input circuit and an output circuit, a feed-back circuit connectedbetween said output circuit andA said input circuit, said feed-back ciruit including a delay network for reversing the phase of the feed-back voltage and also including a cathf ode coupled amplifier tube whichhas a cathode resistor coupled to the input terminals of said network, the input terminals of said network having a Acertain direct-current potential component thereacross in the absence of changes in the amplitude of the oscillations applied thereto,

and means for controlling the gain of said cathode coupled tube in accordance with a modulating signal while simultaneously holding the direct-'current potential across the input terminals i of the delay network substantially constant, said is supplied to the anodes of said tubes, said moduof said last amplifier tube. y

11. The invention according to claim 10Iwherein the oscillator includes an additional feed-back connection which has a time constant that .is of the orderv of the time period of one quartericycle of said oscillations.

12. An oscillator of the resistor-capacitor` type comprising an amplifier tube havingandnput circuit and an output circuit, a feed-back circuit connected between said output.' circuit and said input circuit, said feed-back circuit including a delay network for reversing the phase ofY the i feed-back voltage and also including aV cathode coupled amplier tube which-has a cathode resistor coupled to the input terminals of said network, the input terminals of said network having a certain direct-current potential component thereacross in the absence of changes in the amplitude of the oscillations applied thereto, land means for controlling the gain of said cathode coupled tube in accordance with a modulating signal while simultaneously holding. the directlating signal being applied tothe input `circuit and having a value substantiallylessthan that.

of the-other cathodefresistor.

13. The invention according to `claim 12 whereinthe oscillator includes an additional feed-back connection whichv has a time constant substan-v tially equal tothe oscillation build-up .time of the oscillator and wherein the said modulating signal consists of keying dots ofA variable'width for starting and stopping said oscillations.

14. An oscillator of the resistor-capacitor type comprising an oscillator tube" having lan input circuit and an output circuit, a feed-back circuit connected between said output circuit and said input circuit, said feed-back circuit including a delay network for reversing the phase of input terminals of the delay network substan tially constant, said last means comprising a rectiiier connected between the anode of said oscillator tube and the anode of said control tube to conduct current in one direction, and a second rectifier connected between the anode of said oscillator tube and the cathode of said control tube to conduct current in the opposite direction. f

MAURICE ARTZT. 

